The E2 Elimination Reaction

Updated February 16 th 2004

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Objective

This reaction illustrates the base-induced dehydrohalogenation of alkyl halides with strong base and is used extensively for the preparation of alkenes.  The stereo and regiochemical effects of the size of the base is investigated, and the product mixture is analyzed by the use of gas chromatography.

Grading

You will be assessed on:

  • Successful completion of the elimination reaction
  • Write-up in your Lab Notebook (see Lab Notebook Guidelines) and answers to the Post-Lab Questions
  • TA evaluation of lab procedure

Prior Reading:

Technique 1:  Gas Chromatography (pp. 44-49)

Technique 7:  Collection or Control of Gaseous Products (pp. 90-92)

Reaction

Experimental Procedure

Introduction
Base-induced elimination (dehydrohalogenation) of alkyl halides is a general reaction and is an excellent method for preparing alkenes.  This process is often referred to as E2 elimination, since a hydrogen atom is always removed b to the halide (leaving group):

A high concentration of a strong base in a relatively nonpolar solvent is used to carry out the dehydrohalogenation reaction.  Such combinations as sodium methoxide in methanol, sodium ethoxide in ethanol, potassium isopropoxide in isopropanol, and potassium tert-butoxide in tert-butanol or dimethyl sulfoxide (DMSO) are often used.

Elimination reactions almost always yield an isomeric mixture of alkenes, where this is possible.  Under the reaction conditions, the elimination is regioselective and follows the Zaitsev rule when more than one route is available for the elimination of HX from an unsymmetrical alkyl halide.  That is, the reaction proceeds in the direction that yields the most highly substituted alkene. For example,

In cases where cis or trans alkenes can be formed, the reaction exhibits stereo selectivity, and the more stable trans isomer is the major product.

Experimental evidence indicates that the five atoms involved in the E2 elimination reaction must lie in the same plane; the anti-periplanar conformation is preferred.  This conformation is necessary for the orbital overlap that must occur for the p bond to be generated in the alkene.  The sp3-hybridized atomic orbitals on carbon that comprise the C-H and C-X s  bonds broken in the reaction develop into the p orbitals comprising the p bond of the alkene formed:

There is a smooth transition between reactant and product.  Analogous to the SN2 reaction, no intermediate has been isolated or detected.  Furthermore, no rearrangements occur under E2 conditions.  This situation is in marked contrast to E1 elimination reactions, where carbocation intermediates are generated and rearrangements are frequently observed.

The alkyl halide adopts the anti-periplanar conformation in the transition state, and experimental evidence demonstrates that if the size of the base is increased, then it must be difficult for the large base to abstract an internal b-hydrogen atom.  In such cases, the base removes a less hindered b -hydrogen atom, leading to a predominance of the thermodynamically less stable (terminal) alkene in the product mixture.  This type of result is often referred to as anti-Zaitsev or Hofmann elimination.  Thus, in the reaction of 2-bromo-2,3-dimethylbutane given above, the 2,3-dimethyl-1-butene would be the major product (anti-Zaitsev) if the conditions used uses a bulkier base.

The periplanar arrangements are illustrated in the Newman projections below:

Dehydrohalogenation of alkyl halides in the presence of strong base (E2) is often accompanied by the formation of substitution (SN2) products. The extent of the competitive substitution reaction depends on the structure of the alkyl halide.  Primary alkyl halides give predominantly substitution products (the corresponding ether), secondary alkyl halides give predominantly elimination products, and tertiary alkyl halides give exclusively elimination products. For example, the reaction of 2-bromopropane with sodium ethoxide proceeds as follows:

In general, for the reaction of alkyl halides with strong base,

Physical Properties of Reactants:

Compound

MW

Amount

mmol

bp (°C)

D

np

2-Bromobutane

137.03

100 mL

0.92

91.2

1.26

1.4366

Methanol

32.04

3.5 mL

 

64.9

0.791

1.3288

2-Propanol

60.09

3.5 mL

 

82.4

0.785

1.3776

2-Methyl-2-propanol (tert-butanol)

74.12

3.5 mL

 

82-83

0.786

1.3838

3- Ethyl-3-pentanol

116.20

3.5 mL

 

140-142

0.839

1.4266

Sodium

22.98

60 mg

2.6

883

0.97

 

Potassium

39.10

60 mg

1.5

760

0.86

 

Reagent Combinations:

Alcohol Solvent
Metal
Alkoxide Base Produced

Methanol

Sodium

Sodium methoxide

2-Propanol

Potassium

Potassium 2-propoxide

2-Methyl-2-propanol (tert-butanol)

Potassium

Potassium 2-methl-1-2-propoxide (potassium tert- butoxide)

3-Ethyl-3-pentanol

Potassium

Potassium 3-ethyl- 3-pentoxide

Reagents and Equipment . The combinations of reagents in Table 6.5 may be used to prepare the alkoxide base.  Students should compare results to observe a total picture of the effect.

Preparation of the Alkoxide Base. Measure and add to a 5.0 mL conical vial containing a magnetic spin vane 3-3.5 mL of the anhydrous alcohol to be used (see Table above).  Add a 60-mg piece of potassium (or sodium) metal and immediately attach the vial to a reflux condenser protected by a calcium chloride drying tube.  Place the arrangement in a sand bath and with stirring heat the mixture gently (~50 °C).

NOTE. If the sodium/methanol combination is used, it is not necessary to heat the mixture.  A fairly vigorous reaction occurs at room temperature.  It is recommended that the instructor cut the Na/K metal before commencing the laboratory.

CAUTION: Handle sodium and potassium with care.  These metals react vigorously with moisture and are kept under paraffin oil or xylene.  Remove the small piece of the metal from the oil using a pair of forceps or tongs-never use your fingers!  Dry the metal quickly by pressing it with filter paper (to soak up the oil), and immediately add it to the alcohol in the reaction vial.  Any residual pieces of sodium/potassium should be stored in a bottle marked “sodium/potassium residues."  Never throw small pieces of these metals in the sink or in water.  There will be a large beaker of methanol for you to dispense your unused metal into.  To destroy, in the hood, add small amounts of the metal to absolute ethanol.

When all the metal has reacted, remove the assembly from the sand bath and cool to near room temperature (do not remove the drying tube.).

 

Table 6.6       Temperature Conditions

Base

Temperature (°C)

NaOCH3

100-110

KOCH(CH3)2

130-140

KOC(CH3)3

140-150

KOC(CH2CH3)3

175-180

 

Reaction Conditions. Remove the drying tube from the condenser and use a calibrated Pasteur pipette to introduce 100mL of 2-bromobutane down through the condenser into the vial. Replace the drying tube and place the assembly in the preheated sand bath (see Table 6.6). Remove the drying tube and attach the gas delivery tube to the top of the condenser so that the open end of the tube is beneath the water level of the reservoir. lf the connection to the top of the condenser is not made with an O-ring cap seal connection, lightly grease the ground-glass joint to insure a gas-tight seal. After about 10-15 air bubbles emerge, place the water-filled gas collection tube over the open end of the gas delivery tube.

Isolation of Product. Collect about 6­-7 mL of gas in the collection reservoir and then use a hypodermic syringe to withdraw a 0.7­ to 0.8-mL sample through the rubber septum for GC analysis.

NOTE. Remove the gas delivery tube from the collecting reservoir and then from the water before discontinuing the heat on the reaction vial. This order of events prevents water from being sucked back into the reaction flask.

Purification and Characterization. The collected gas is analyzed by gas chromatography without further purification.

                    

Post Lab Questions

1.         Predict the more stable alkene of each of the following pairs:

(a) 1-Hexene or trans-3-hexene

(b) trans-3-Hexene or cis-3-hexene

(c) 2-Methyl-2-hexene or 2,3-dimethyl-2-pentene

2.         Starting with the appropriate alkyl halide and base-solvent combination, outline a synthesis that would yield each of the following alkenes as the major or only product and include your reasoning:

(a) 1-Butene

(b) 3-Methyl-1-butene

(c) 2,3-Dimethyl-1-butene

(d) 4-Methylcyclohexene

3.         When cis-1-bromo-4-tert-butylcyclohexane reacts with sodium ethoxide in ethanol, it reacts rapidly to yield 4-tert -butylcyclohexene.  Under similar conditions, trans-1-bromo-4-tert-butylcyclohexane reacts very slowly.  Using conformational structures, explain the difference in reactivity of these cis-trans isomers.

Bibliography

Several dehydrohalogenation reactions of alkyl halides using alkoxide bases are given in Organic Syntheses:

1.         Allen, C. F.; Kalm, M. J.  Organic Syntheses; Wiley: New York, 1963; Collect. Vol. IV, p. 398.

2.         McElvain, S. M.; Kundiger, D. Organic Syntheses; Wiley: New York, 1955; Collect. VoI. III, p. 506.
3.         Paquette, L.A.; Barrett, J. H. Organic Syntheses; Wiley: New York, 1973; Collect. VoI. V, p. 467.
4.         Schaefer, J. P.; Endres, L. Organic Syntheses; Wiley: New York, 1973; Collect. Vol. V, p. 285.

For an overview of elimination reactions: March, J. Advanced Organic Chemistry, 4th ed.; Wiley New York, 1992, p. 982.

This experiment  is adapted from the method given by: S. A. Leone; J. D. Davis J Chem. Educ. 1992, 69, A175.